Engines, such as those which power aeronautical and industrial equipment, may employ a compressor to compress air that is drawn into the engine and a turbine to capture energy associated with a combustion of a fuel-air mixture. The turbine typically includes alternating stages of rotatable and static/non-rotatable structure. The rotatable structure is frequently implemented as a hub/disk coupled to a shaft, with a multiple of blades that are seated by the disk. The blades extend radially from the disk to a distal end/tip. Disposed (e.g., radially disposed) between an engine structure (e.g., case) and the blade tips is a seal that is frequently referred to in the art as a blade outer air seal (BOAS).
The gap/distance between the blade tips and the BOAS, referred to herein as a blade tip clearance (BTC), is subject to change during the operation of the engine. For example, differential thermal growth, engine vibrations/deflections, etc., may cause the BTC to increase or decrease over an operational envelope/profile of the engine. If the BTC is too small, a knife edge associated with the blade tip may cut/dig into the BOAS causing undesirable/premature wear of the BOAS and/or the knife edge. On the other hand, the BTC represents a leakage path with respect to the fuel-air mixture; a large valued BTC leads to inefficiencies (e.g., a large valued BTC reduces fuel efficiency of the engine). Accordingly, the BTC is a parameter that is the subject of great interest.
During, e.g., engine development and/or test, one or more probes may be used to determine what the BTC is for one or more of the blades. It can be difficult to accurately measure the BTC. For example, when a knife edge shifts axially relative to, e.g., the BOAS, the knife edge may not be in the field of view of a probe unless the probe is configured with a sufficiently large field of view. However, if the field of view is too large the probe may obtain reflected energy from, e.g., a blade shroud that is present in the background. The contribution of reflected energy changes the calculated value of the BTC significantly based on axial shifts/displacements of the knife edge, such that the generated BTC may be invalid/incorrect.
Accordingly, what is needed is an ability to characterize an axial shift of the blades/associated knife edge. Furthermore, what is needed is an ability to characterize/determine the BTC with enhanced precision.